1. Field of the Invention
The present invention relates to an image forming apparatus and a droplet ejection control method, and more particularly to a recording control technology for an image forming apparatus for forming images by means of dots formed on a recording medium.
2. Description of the Related Art
Recently, inkjet recording apparatuses (inkjet printers) have become common as recording apparatuses for printing and recording images captured by digital still cameras, and the like. An inkjet recording apparatus comprises a plurality of recording elements (nozzles) in a head, the recording head being move to scan a recording medium while droplets of ink are ejected onto the recording medium from the recording elements, the recording medium being conveyed through a distance corresponding to one line, each time one line of an image is recorded onto a recording medium, and an image being formed onto the recording paper by repeating this process.
Inkjet printers include those which use a fixed-length serial head, and carry out recording by moving the head in the lateral direction of a recording medium, and those which use a line head in which recording elements are arranged over a length corresponding to the full dimension of one edge of the recording medium. In a printer using a line head, it is possible to record an image across the entire surface of the recording medium, by scanning the recording medium in an orthogonal direction to the direction in which the recording elements are arranged. In a printer using a line head, it is not necessary to provide a conveyance system, such as a carriage, for moving a short-dimension head, nor is it necessary to move a carriage, or perform complicated scanning control of the recording medium. Furthermore, since only the recording medium is moved, it is possible to increase the recording speed in comparison to printers using serial heads.
In an inkjet printer, one image is represented by combining dots formed by ink ejected from recording elements (nozzles). High image quality can be achieved by making the dots small in size and by using a large number of pixels per image. Small dot size can be achieved by reducing the amount of ink ejected, and therefore it is necessary to control the ink ejection volume finely and accurately. The relative speed of the recording medium and the recording head, and the ink ejection timing, are controlled in such a manner that adjacently positioned dots are deposited at prescribed positions.
Japanese Patent Application Publication No. 3-247450 discloses an inkjet recording apparatus and proposes a technology for calculating printing times for avoiding image distortion, according to the ink absorption characteristics, ink permeability, ink (dot) density, ink volume, ink evaporation characteristics, and ambient temperature. In other words, the drying time of the ink, and the like, is estimated from the aforementioned parameters, and the interval between one recording operation and the next is adjusted accordingly.
Furthermore, Japanese Patent Application Publication No. 10-250059 discloses a method of manufacturing a recording head for an inkjet printer, and a printing method; more specifically, a method of manufacturing a print head in which the distance between nozzles is determined in accordance with the ink drying time, and a print method using this print head.
However, if a plurality of dots are ejected so as to land in an overlapping manner on a recording medium, the ink droplets combine, the original circular shape of the dots are lost, and hence it becomes difficult to form the desired image.
In the inkjet recording apparatus disclosed in Japanese Patent Application Publication No. 3-247450, the time for avoiding image distortion is determined for each parameter, but it is extremely difficult to determine a time for avoiding image distortion in relation to various types of dot patterns. Furthermore, no sufficient explanation of the recording method is given in cases where mixed patterns combining different dot pitches and dot sizes are formed in one image.
In the method for manufacturing a recording head for an inkjet printer and the printing method disclosed in Japanese Patent Application Publication No. 10-250059, it is stated that a second dot is ejected so as to land after the drying time for a first dot has elapsed. However, since an adjacent dot cannot be deposited onto the paper until the existing dot on the surface of the paper has dried completely, then printing takes an extremely long time. Furthermore, since the distance between the nozzles is determined by estimating the drying time based on one set of conditions, and since this is a fixed value, it is not possible to respond to cases where different ink or paper is used.